To explore the effects of the salinity gradient on the abundance and diversity of ammonia-oxidizing microorganisms in coastal salinized soils, salinized soil samples with different salinities (6.4‰~51.3‰) were collected from four estuary transects (Yellow River, Bailang River, Di River, and Jiaolai River) in the southwest coast of Laizhou Bay. The methods of soil chemistry and molecular ecology (qPCR and T-RFLP) were applied to analyze the activities, abundance and community structures of ammonia-oxidizing archaea (AOA) and bacteria (AOB). The results showed that soil nitrification potential (NP) was significantly inhibited at high salinity (34.7‰-51.3‰) and it was significantly correlated with soil salinity, pH, and NO₃^--N level. Abundance of AOA-amoA gene was two orders of magnitude higher than that of AOB-amoA gene, which was highest at medium salinity (9.92×10⁶ copies/g soil), and was inhibited at high salinity (5.28×10⁶ copies/g soil, P<0.05). Abundance of AOB-amoA was correlated with soil salinity, which was highest at low salinity and significantly reduced at medium and high salinities. However, the diversities and community structures of both AOA and AOB were little affected by the salinity gradient based on the T-RFLP data. Furthermore, correlation analysis showed that NP was not significantly correlated with abundance of AOA or AOB, but negatively significantly correlated with the ratio of AOA/AOB and the Shannon index of AOA. In conclusion, the ammonia oxidation activity and its functional microorganisms in coastal saline soils were affected intensively by the fluctuation of salinity. The changes of soil nitrogen availability caused by salinity and pH may be the key factors affecting nitrification activity, microbial abundance and community composition in coastal saline soils.